<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<script type="text/javascript" src="js/jquery-1.12.4.min.js"></script>
	<script type="text/javascript">
		
		$(function(){

			var $div = $('.box');

			var sTr = $div.css('fontSize');

			alert(sTr);

			$div.addClass('big');


			$div.removeClass('red');






		})

	</script>
	<style type="text/css">
		.box{
			width: 400px;
			height: 100px;
			margin:50px auto 0;
			background-color: pink;
		}

		.big{
			font-size: 30px;
		}

		.red{
			color:red;
			text-align: center;
			line-height: 100px;
		}
	</style>
</head>
<body>
	<div class="box red">这是一个div</div>
</body>
</html>